In indirectly driven fuel injectors, an actuator, in particular a piezo-actuator, controls the valve piston of a control valve with which the pressure ratios between the control space and the valve space are influenced. The movement of the control valve piston is determined here by the respectively prevailing force conditions (pressure in the control space and actuator space as well as the force/expansion brought about by the actuator). If an injection is to be triggered, electrical energy is applied to the actuator. The actuator can be actuated here, for example, in an energy-controlled fashion. For this, the charge current and the voltage at the actuator are measured in parallel and the current energy is determined according to the formula E=0.5*(I_PIEZO)dt*U_PIEZO.
In the case of a piezo-actuator, the actuator expands owing to the piezo-electric effect and applies a force to the valve piston of the control valve. If the actuator force exceeds the opposing force, the control valve opens and the pressure in the control space is reduced. The force conditions at the valve needle change as a function of the ratio of the fuel quantity running into the control space and the fuel quantity running out of the control space, said valve needle moving in accordance with the resulting force and clearing the injection holes. In order to end the
injection, the actuator is discharged after a predetermined time and the control valve is closed. The pressure which builds up in the control space closes the injection valve by means of the movement of the valve needle.
The quantity tolerances of the individual injectors, in particular over their service life, are influenced here, in particular, by mechanical tolerances, the temperature, the run-in behavior and the wear. As a result, a switching leak can occur in the corresponding injector. The energy requirement of the actuator, in particular piezo-actuator, which is required to open the control valve plays a central role in the size of the respective quantity tolerances. This energy requirement corresponds to a stroke which is to be overcome by the actuator and which is referred to as an idle stroke. For this reason, the ACTUAL INJECTION differs from the SETPOINT INJECTION as a result of changes to this idle stroke due to wear, service life, temperature, load profile, etc.